Technical Field
Embodiments of the invention relate generally to power generation and, more particularly, to a system, method and apparatus for maintaining a pressure balance and for controlling the flow of solids in a solids flow loop of a power generation system.
Discussion of Art
Fluidized bed combustion (FBC) is a combustion technology used in power plants, primarily to burn solid fuels. FBC power plants are more flexible than conventional power plants in that they can be fired on coal, coal waste or biomass, among other fuels. The term FBC covers a range of fluidized bed processes, including circulating fluidized bed (CFB) boilers, bubbling fluidized bed (BFB) boilers and other variations thereof. In an FBC power plant, fluidized beds suspend solid fuels on upward-blowing jets of air during the combustion process in a combustor, causing a tumbling action which results in turbulent mixing of gas and solids. The tumbling action, much like a bubbling fluid, provides a means for more effective chemical reactions and heat transfer in the combustor.
During the combustion process of fuels which have a sulfur-containing constituent, e.g., coal, sulfur is oxidized to form primarily gaseous SO2. In particular, FBC reduces the amount of sulfur emitted in the form of SO2 by a desulfurization process. A suitable sorbent, such as limestone containing CaCO3, for example, is used to absorb SO2 from flue gas during the combustion process.
To further increase utilization of the fuel and efficiency of sulfur capture, a cyclone separator is typically used to separate solids, e.g., unutilized fuel and/or limestone, entrained in flue gas leaving the combustor. The separated solids are then returned to the combustor via a recirculation means, e.g., a recirculation pipe, to be used again in the combustion process. A sealpot, sometimes referred to as a “j-valve,” maintains a seal between the combustor and the cyclone separator to prevent unwanted escape of flue gas from the combustor backward, e.g., in a direction opposite to flow of the separated solids into the combustor, through the recirculation pipe.
In connection with the above, maintaining a pressure balance on opposite sides of the sealpot (i.e., between a dipleg/standpipe and riser) is critical for maintaining a positive flow of solids back to the combustor and preventing backflow through the sealpot. In addition, maintaining such pressure balance is necessary to prevent compressive or expansive forces on the transport airbed of the sealpot, which could inhibit precise flow control of the solids, leading to overall system inefficiencies.
Similar considerations are also necessary in chemical looping systems. Chemical looping systems utilize a high temperature process, whereby solids such as calcium or metal-based compounds, for example, are “looped” between a first reactor, called an oxidizer, and a second reactor, called a reducer. In the oxidizer, oxygen from air injected into the oxidizer is captured by the solids in an oxidation reaction. The captured oxygen is then carried by the oxidized solids to the reducer to be used for combustion and/or gasification of a fuel such as coal. After a reduction reaction in the reducer, the solids products from reactions including unused oxygen carrier, are returned to the oxidizer to be oxidized again, and the cycle repeats. In such systems, a sealpot may be utilized to preserve a required pressure seal and prevent an undesired pressure differential that could cause backflow, as discussed above. For example, a sealpot may be utilized in between the output of the oxidizer and the input of the reducer to provide a flow of oxidized solids to be transported to the reducer and prevent backflow therefrom. Like the systems described above a pressure balance must be maintained on opposite sides of the sealpot to maintain positive solid flow and to prevent backflow of solids.
With existing systems, however, flow control of the solids has proven challenging, impacting overall system performance and efficiency. In particular, it has been difficult to avoid clogging of the solids and gas entrained without losing control over the flow of the solids within the sealpot and from the sealpot. In view of the above, there is a need for a system, apparatus and method for balancing the pressure between a seal leg and a riser leg of a solids flow loop in order to more precisely control the flow of solids therethrough.